1. Field of the Invention
The invention relates to an air-fuel ratio control apparatus that makes the actual air-fuel ratio substantially equal to the target air-fuel ratio.
2. Description of the Related Art
In internal combustion engines, air-fuel ratio control for making the actual air-fuel ratio substantially equal to the target air-fuel ratio is performed. For example, Japanese Patent Application Publication No. JP-A-10-103138 describes a known internal combustion engine in which air-fuel ratio control is performed.
In the air-fuel ratio control, a feedback correction value that compensates for a temporary deviation of the actual air-fuel ratio from the target air-fuel ratio is calculated through the air-fuel ratio feedback control, and an air-fuel ratio learned value that compensates for a constant deviation of the actual air-fuel ratio from the target air-fuel ratio is calculated through the air-fuel ratio learning control. Then, the final fuel injection amount is calculated in consideration of the feedback correction value and the air-fuel ratio learned value.
In the internal combustion engines, if the fuel injected from an injector is not sufficiently atomized, such fuel which is not sufficiently atomized is mixed into the lubricating oil. As a result, the lubricating oil is diluted with such fuel, namely, “fuel-dilution” occurs. When the fuel is vaporized from the lubricating oil as the temperature of the lubricating oil increases, and is supplied to a combustion chamber again through a blow-by gas reductor, etc., the air-fuel ratio changes due to the influence of such fuel.
If the actual air-fuel ratio becomes constantly richer than the target air-fuel ratio due to the fuel vaporized from the lubricating oil and supplied to the combustion chamber, the air-fuel ratio learned value is updated to a value that decreases the fuel injection amount (i.e., a value that changes the air-fuel ratio to a value leaner than the target air-fuel ratio). Also, as the actual air-fuel ratio becomes richer than the target air-fuel ratio by a larger amount, the fuel injection amount is corrected by a larger amount, using the air-fuel ratio learned value.
When the air-fuel ratio learned value is set to such a value, if the internal combustion engine is started while the temperature thereof is excessively low, the following problem occurs. Although the fuel is not vaporized from the lubricating oil due to the excessively low temperature of the internal combustion engine, the air-fuel ratio learned value is set to a value that decreases the fuel injection amount. As a result, the actual air-fuel ratio becomes excessively lean, causing misfire.